In magneto-optical recording, digital data in the form of magnetic domains are written on a magnetic thin film having perpendicular magnetic anisotropy by a laser beam which raises the temperature of the focussed area of the film up to the Curie temperature under an applied external magnetic field of about 300 Oe. This information is read by irradiating the medium with a linearly polarized beam and detecting the change of polarization of the incident beam caused by the interaction with the magnetization.
The currently used recording media are made of thin films of rare earth-transition metal alloys. The widely accepted choice today is an alloy of TbFeCo. Though these alloy thin films are widely used today, they face limitations of (a) low Kerr rotation at blue wavelengths and hence are unsuitable for future high-density storage (b) low shelf life time due to their easy oxidation.
The multilayers of ferromagnetic metal and noble metal are being explored today as ideal alternatives for high-density storage media. These multilayers for application to magneto-optical storage media are expected to satisfy certain requirements such as (a) perpendicular magnetic anisotropy(PMA) (b) large Kerr rotation, especially at lower wavelengths, (c) large room temperature coercivity,(d) low Curie temperature in the range 150-250.degree. C. and (e) corrosion-resistance.
Multilayers of Co/Pt and Co/Pd with novel properties of large perpendicular magnetic anisotropy and relatively high Kerr rotation at short wavelengths are being considered as ideal choices for new high density magneto-optical recording media. Co/Pt or Co/Pd multilayers with cobalt thickness, t.sub.Co, of about 3 or 4 .ANG. have been found to satisfy most of the desired magnetic and magneto-optical properties.
However, lower Curie temperatures not only reduce the power of lasers required for writing but also decide the write/erase cyclability. And yet, the Curie temperatures of these multilayers, Co/Pt and Co/Pd are relatively higher by about 100-200.degree. C. than the writing temperature achievable practically for these applications today.
Alloying cobalt layer with nickel in Co/Pt multilayers has been found to a good alternative to reduce Curie temperature. However, alloying cobalt layer faces limitations of associated changes in saturation magnetization, Kerr rotation, changes in interfacial structure, and decrease in anisotropy energy etc.
Though there are published reports of observation of perpendicular magnetic anisotropy in Ni/Pt (Krishnan et.al., Appl.Phys.Lett. 59 (1991), pp.3649-3650), all of these reports indicate observation of this essential feature at temperatures far below room temperature, about 5 K, which make them unsuitable for application to magneto-optical recording media.